The objective of this project is to define the initial, intra-cellular events of steroid hormone action. Synthetic glucocorticoid derivatives, some of which could react to form covalently labeled receptors via affinity labeling, are being used with Glucocorticoid-responsive rat hepatoma tissue culture cells to examine: (1) steroid-receptor binding site interactions; (2) the effects of steroid binding on receptor conformation; and (3) the nature of "activation" of receptor-steroid complexes. In the course of preparing these synthetic glucocorticoids, we have uncovered some new steroid chemistry, i.e., a new, base-catalyzed cleavage of C21-steroids to give 17-keto-steroids and a facile, high yield conversion of C-17 Alpha-keto-mesylates of glucocorticoids to spiro C-17 oxetan-3-ones. One of these synthetic steroids (dexamethasone-21-mesylate (DM)) is a potent, irreversible antiglucocorticoid. Further experiments indicate that DM is the first affinity label for the steroid binding site of glucocoticoid receptors. A patent has been filed to cover DM and related compounds.